It is known that hydroxyl-terminated polymers, when crosslinked by using a compound having a functional group reactive with a hydroxyl group, for example an isocyanate compound, as a curing agent, give cured products excellent in heat resistance and durability, among others.
The principal chain skeleton of such hydroxyl-terminated polymers includes, among others, polyether polymers such as polyethylene oxide, polypropylene oxide and polytetramethylene oxide; hydrocarbon polymers such as polybutadiene, polyisoprene, polychloroprene, polyisobutylene and hydrogenation products thereof; and polyester polymers such as polyethylene terephthalate, polybutylene terephthalate and polycaprolactone. According to their principal chain skeleton and crosslinking manner, they are used for various applications.
Unlike the above-mentioned polymers, which are obtainable by ionic polymerization or condensation polymerization, hydroxyl-terminated vinyl polymers obtained by radial polymerization have scarcely been put into practical use.Among vinyl polymers, (meth)acrylic polymers have those characteristics which can hardly be attained by the above-mentioned polyether polymers, hydrocarbon polymers or polyester polymers, for example high weather resistance and transparency. Ones having hydroxyl groups at the side chains are used in weather-resistant paints, for instance.
(Meth)acrylic polymers having hydroxyl groups at the chain ends, if obtained in a simple and easy manner, will be able to give cured products improved in physical properties such as elasticity compared with ones having hydroxyl groups at the side chains. Therefore, a number of investigators have so far tried to establish production processes therefor. However, it is not easy to produce them on a commercial scale.
Japanese Kokai Publication Hei-05-262808 discloses a method for synthesizing (meth)acrylic polymers having a hydroxyl group at each terminus by using a hydroxyl-containing disulfide as a chain transfer agent. For securing hydroxyl groups introduction into both ends, however, this method requires use of the chain transfer agent in a large amount relative to an initiator. This poses a problem from the viewpoint of production step. In addition, Japanese Kokoku Publication Hei-01-19402 discloses a method for producing hydroxyl-terminated (meth)acrylic polymers by using hydrogen peroxide as an initiator. It is difficult, however, to secure hydroxyl groups introduction into both ends. Thus, a method actually employed consists in copolymerization with a hydroxyl-containing vinyl monomer (e.g. 2-hydroxyethyl methacrylate). Moreover, Japanese Kokai Publication Hei-04-132706 discloses a method for producing a hydroxyl-terminated vinyl polymer which comprises preparing a halogen-terminated (meth)acrylic polymer by polymerizing a (meth)acrylic monomer using a telogen such as carbon tetrachloride and then substituting the terminal halogen by reacting with a nucleophile such as a diol compound, hydroxyl-containing carboxylic acid or hydroxyl-containing amine. By this method, too, it is difficult to introduce a functional group into both termini at high ratios, since the chain transfer of the telogen is not sufficient.
Accordingly, it is an object of the present invention to provide a method for producing a (meth)acrylic polymer which contains terminal hydroxyl groups in a higher proportion as compared with the prior art methods, as well as a curable composition containing the same as a main component thereof. Another object of the invention is to provide a method for further introducing another functional group (e.g. alkenyl, crosslinkable silyl) by making use of the reactivity of the terminal hydroxyl groups, as well as a curable composition containing the same.